The invention relates to a rotary tightening device for a safety belt, particularly in motor vehicles, having a belt retractor, whose belt take-up shaft upon triggering of the rotary tightening device coupled thereto is rotated in the take-up direction of the safety belt, there being associated with the belt take-up shaft, as a drive, a driving wheel in whose plane the housing of the belt retractor forms a channel, which embraces the driving wheel over at least part of its periphery, for the through passage of mass bodies acting as driving means upon the driving wheel and lying loosely adjacent to one another in the channel.
The basic construction of a rotary tightening device having the features mentioned above is described in DE 29 31 164 A1; in the known rotary tightening device, the belt take-up shaft is provided with a driving wheel, the housing in the plane of the driving wheel having a channel which embraces the driving wheel over part of its periphery; in said channel, mass bodies are driven under the action of a pyrotechnic drive out of a tube disposed tangentially to the driving wheel or the channel and as a result of their friction-locking connection to the driving wheel, which for said purpose is provided with a circumferential contoured groove, set said driving wheel in rotation as they run through the channel. According to an embodiment, the mass bodies may take the form of balls made of a suitably hard plastics material. An outlet opening for the mass bodies may be provided at the end of the channel, the mass bodies exiting from the channel through said outlet opening and passing into a suitably provided receiving container.
A drawback of said known rotary tightening device is that the drive is effected through friction locking; consequently, on the one hand, considerable energy is lost and, on the other hand, it is difficult to tune the contoured groove and the mass bodies to one another in such a way as to produce a defined friction lock and prevent the mass bodies from becoming wedged or sliding through without power transmission. A further drawback associated with the known rotary tightening device is that-when the hard mass bodies encounter the driving wheel, because the channel for sealing reasons has only slight clearance, a mutual blocking between the periphery of the driving wheel and the mass bodies may arise in the channel, thereby impairing the efficiency of the drive in the event of triggering.
The object of the invention is therefore to improve a rotary tightening device having the features described initially in such a way that the transmission of the drive power, which is imparted to the mass bodies, to the tightening rotation of the belt take-up roller is improved.